128 



<THE TROPICAL AGRICULTURIST. 



[August i, 188 4 



COCHINEAL COLOURING. 



From what has already been said under the chemical 

 history of cochineal, the rationale of the processes for pre- 

 paring the majority of liquid colourings, so extensively used 

 in confectionery and other operations, will be understood 

 without difficulty. The colouring principle, as has already 

 been shown, is soluble in water, but less so in alcohol — the 

 stronger the alcohol the less capable is it of holding the 

 carmine in solution. As, however, the alcohol is an ex- 

 pensive solvent, and as it presents no special advantages 

 over the aqueous solution in the large majority of uses 

 to which cochineal colouring is applied, it is seldom, if 

 ever, resorted to in common use. 



An aqueous infusion (prepared with heat) of about 1 part 

 cochineal to 10 or 15 of water, and concentrated by evapor- 

 ation to a proper degree, will answer for the majority of 

 pharmaceutical preparations, such as Parrish's syrup, much 

 better than any other colourings into which are introduced 

 chemical compounds which often produce unexpected and 

 generally vexatious complications. For most purposes, how- 

 ever, such a colouring is not bright enough, and recourse 

 is, therefore, had to various means to produce a more 

 brilliant preparation. Practically, by whatever means 

 effected the resulting liquor is merely a solution ul carmine, 

 and the method employed to produce it will probably be 

 determined more by the purposes for which the colouring 

 is required, or by such considerations as convenience or 

 expense, than from any great difference in the resulting 

 product. Consequently, in making cochineal colouring we 

 may either proceed to make it from the carmine direct, 

 or we may make a soluble carmine from the cochineal 

 itself. The former is accomplished generally by means of 

 the solubility of carmine in ammonia, and all that is 

 necessary is to digest the carmine in ammonia water 

 sufficient to effect solution, diluting the resulting solution 

 with water, and adding some preserving media, such as 

 spirit or sugar, &c. An old form for a colouring of this 

 kind, and one which gives as good results as can possibly 

 be obtained working in this direction, is as follows: — 



Carmine, pure, 1 oz. ; solution of ammonia, 6 oz. or q.s.; 

 macerate for a few days, with frequent agitation, and, 

 when dissolved, heat gently so as to drive off excess of 

 ammonia without reprecipitatiug the carmine. Dilute this 

 solution with water, and add sugar, 1J lb; rectified spirit, 

 4 oz.; water sufficient to make the whole measure 40 oz. 

 The two main objections to all such colouring solutions 

 are first, the cost of production, it being quite apparent 

 that the initiatory process of producing such a valuable 

 pigment as carmine adds considerably to the expense of 

 the finished product; and, second, the action that all acids 

 have upon them in shortly reprecipitatiug the carmine. 



Proceeding from the crude cochineal, no process is better 

 known, or has been longer in use, than that of digesting 

 with the aid of heat equal parts of cochineal, salt of tartar, 

 alum, aud cream of tartar with about eight parts of water. 

 The process is so well known that we do not require to 

 give it in full, but, inasmuch as complaints are not infre- 

 quent (and this not without cause, as we will immediately 

 show) of difficulties in working the process, as well as in 

 keeping the product, we may be excused for touching 

 upon several details bearing upon these points. Alum, it 

 is well known, is decomposed in presence of alkaline car. 

 Donates, the hydrate being thrown down with evolution of 

 carbonic acid. When, therefore, a solution of cochineal is 

 treated with salt of tartar and alum, the hydrate of alum- 

 ina is liberated, and seizes upon the colouring matter, for 

 which it has a powerful affinity, and would ultimately throw 

 it down as an insoluble compound, known commercially as 

 carmine lake, but which might chemically be described as 

 impure aluminum carmiuate. On the addition, however, of 

 the cream of tartar, brisk effervescence again takes place, 

 with resolution of the colouring principle, so that the mixture 

 now contains aluminum carminate in solution, plus certain 

 salts the products of decomposition. So much for (he ration- 

 ale of the process, without encumbering the explanation with 

 too much chemistry. Now, if, instead of working with 

 cochineal, we simply take equal parts of the three salts 

 mentioned, and dissolve them in the proportion of water 

 generally taken in this formula for cochineal colouring, we 

 will find that the resulting liquid, after heating to drive 

 off all carbonic acid, will be feeby alkaline, and, further, 

 that it will be supersaturated with the resulting salts. 



Consequently, a slight excess of cream of tartar, a cirt urn- 

 stance which might happen through carelessness in w« igh- 

 ng, or a reduction in the quantity of salt of tartar thiough 

 excess of moisture or other causes which might be men- 

 tioned, wdl materially affect the ultimate result so far as 

 an alkaline or acid reaction is concerned. Not only so, but 

 the liquid being supersaturated will necessarily begin to 

 deposit on cooling, and will continue to deposit with de- 

 creasing temperature. The first observation here made ap- 

 plied to cochineal colouring affects the shade and depth 

 of colour, &c., of the preparation, while the second observ- 

 ation affects its permanency, the precipitation from the 

 supersaturated solution of necessity carrying very much 

 of the colouring principle along with it. Behind this, how- 

 ever, a more important point still remains to 1 e considered. 



Like many other infusions of organic origin, the coch- 

 ineal colouring will not keep without the addition of some 

 agent to prevent putrefactive changes. Chloride of soda 

 has been recommended for this purpose, but it is decidedly 

 objectionable from a chemical point of view. Glycerine is 

 also used ; and, although we have had no experience of its 

 anticeptic properties in this case, our experience in other 

 directions analogous is that, unless added in considerable 

 quantity, it will not arrest decomposition for any length 

 of time. Sugar and alcohol are the agents more commonly 

 employed, and in ordinary circumstances they are certainly 

 the best adapted for the purpose ; but here it is needless 

 to point out that with a solution already saturated they 

 encourage deposition, not oidy of the salts contained hi 

 solution, but of the colourising principle also. We have 

 frequently seen, where these agents have been added, nearly 

 one-half of the finished product lost from the sediment pre - 

 cipitated or thrown down. We only mention these points 

 to give emphasis to the fact already sufficiently well known, 

 namely, that the process is not by any means a perfect 

 one, or without its difficulties in more than one direction. 

 Mr. E. Bother {Pharmacist and Chemist, June 18S0) tried 

 to reduce the alum process to a scientific basis, and iu a 

 wonderfully-elaborate production gave a process which, 

 divested of all its peculiarities of phraseology might shortly 

 be explained as, first, the production of a pure aluminum 

 carminate, and, second, the bringing of this carmiuate into 

 an "available solution by means of a monad salt of a 

 vegetable acid." The process is, shortly, as follows : — One 

 part cochineal is macerated for two days with an eighth 

 part (by measure) of hydrochloric acid, a fourth part of 

 chloride of sodium, and four parts of water. Having occa- 

 sionally stirred the mixture, it is ultimately decanted. The 

 residue is then treated a second time in the same manner, 

 and for the same length of time, with plain water, and 

 a third time with the same quantity of water to which 

 has been added one part chloride of sodium. The maro 

 is then strongly pressed, and the several macerates 

 mixed and allowed to stand until any sediment has sub- 

 sided. To the clear liquid is added one-fourth part 

 alum, and, when dissolved, one-third part solution of 

 ammonia (16 per cent). The resulting precipitate is 

 thoroughly washed with water, and finally dissolved in a 

 solution of citrate of soda, made by dissolving one-eighth 

 part citric acid in four parts water, and adding, with the 

 a ; d of a gentle heat, subcarbonate of soda to saturation. 

 The wholt- is made to measure three-and-a-half parts, and 

 to this one-half part stronger alcohol is added "to obviate 

 septic degeneration." The advantages olaimed for this pro- 

 cess are complete extraction of the colouring priuciple and 

 a great* r concentration of the finished product ; the dis- 

 advantages, we may add, are the time and attention required 

 in its preparation, the certainty of a loss of colouring 

 matter by precipitation on the addition of the alcohol, 

 and, like many other scientific formulas, a product fre- 

 quently uncertain and unsatisfactory. Quite unconscious 

 that Mr. Pother had been working in the same direction, 

 we had made a series of experiments with the view of 

 effecting solution of the impure aluminum carmiuate of 

 the old process by means of the most suitable vegetable 

 acid. In doing this we found that citric acid was in every 

 respect the best agent, aud, in substituting a proper pro- 

 portion of this acid for the cream of tartar of the old 

 process, we found a product could be obtained at once perm- 

 anent and satisfactory. We give the process, with some 

 degree of confidence, as the simplest and best that has 

 yet been given for cochineal colouring: — Finest silver-grain. 



